In radiometers and other radiant signal measurement systems that switch between a reference signal and measured radiant signal, focal plane arrays are sometimes used to form images from radiation received by a reflector antenna. Millimeter wave (MMW) focal plane array radiometers also have been used in many applications to form images based on thermal sensing of radiated microwave energy. The sensitivity of existing radiometer designs, however, has been limited to about 1 deg K, resulting in poor images.
The principle of operation of the radiometric technique is fully described in the literature. The design of a typical radiometer is based on the technique of comparing the level of electromagnetic noise emitted by an unknown source to a reference or stable noise source. This technique and devices were initially proposed by Dicke [R. H. Dicke, “The Measurement of Thermal Radiation at Microwave Frequencies,” The Review of Scientific Instruments, Vol. 17, No. 7, July 1946].
In a Dicke radiometer circuit, the signals from typically an antenna or other source are sampled and compared with signals from a reference source maintained at a known constant temperature. This overcomes some of the problems of amplifier instability, but in general does not alter effects resulting from imperfect components and thermal gradients.
While other types of radiometric devices have been used with some success, the Dicke (or comparison) type of radiometer has been the most widely used for the study of relatively low level, noise-like millimeter wave signals, especially where the noise signals to be examined are often small in comparison to the internally generated noise level within the radiometer receiver. While there are several types of comparison radiometers, one popular type of radiometer for use in the microwave/millimeter wave frequency bands compares an incoming signal to be measured to a standard or calibrated reference noise signal. This type of radiometer compares the amplitude of an unknown noise signal coming from the source to be examined with a known amplitude of a noise signal from a calibration source. This method has been found useful in measuring with considerable accuracy the effective temperature of an unknown source.
In the Dicke or comparison type radiometer, the receiver input is switched between typically the antenna and a local reference signal noise generator. The detected and amplified receiver output is coupled to a phase-sensing detector operated in synchronism with the input switching circuit. The output signal from the radiometer receiver is proportionate to the difference between the temperature of the reference signal source and the temperature of the source viewed by the antenna, inasmuch as the phase-sensing detector acts to subtract the background or internal noise of the receiver.
It should be understood that the Dicke radiometer typically uses a radio frequency (RF) switch coupled between an antenna and a radiometer receiver, allowing the receiver to alternate between the antenna and a known reference load termination. The receiver output is connected to the synchronous detector that produces an output voltage proportional to a difference between the antenna and the reference temperature. Null balance operation for the Dicke radiometer is typically achieved by coupling in noise from a hot noise diode to the antenna port of the RF switch, allowing the system to match the temperature from standard reference loads.
These types of systems typically use time multiplexed circuits with either analog or RF inputting and gain stages. An output is formed as a differential analog level. These circuits could be similar to existing chopper circuits for analog applications, and a Dicke switch for RF applications. These type of circuits can also be used in instrumentation circuits and measurement circuits that evaluate small changes in a signal.
Some switched measurement systems currently in use have been inadequate because the systems do not correctly compensate for gain variations caused by temperature drift from the temperature at which the system was calibrated. Most existing systems use a thermal control system, for example, a Peltier cooler, which adds an extra cost and an extra power usage to the system.
The Dicke switch is an adequate RF measurement system when it operates in the presence of a large DC or noise offset associated with the measurement reading, typically at a smaller scale. The reference reading can be used to null, or subtract-out from the measurement reading. The Dicke switch system typically uses the time multiplexed circuit where the input is switched between the measurement and the reference in a regular pattern. This allows the same circuit hardware to amplify the signals, resulting in identical system gain for both the measurement and the reference. This common gain allows for a linear subtraction of measurement and reference.
One primary application of this type of system is a radiometer, which measures a small amount of noise variation in a large noise offset. The reference reading is calibrated to be offset at the calibration temperature. The radiometer output is supplied to a log power detector circuit and converted to an analog level, which alternates in time between the measurement and the reference. A controller evaluates the analog levels and performs system operations, whether in analog or digital.
In these types of systems for example, the Dicke switch radiometer have several drawbacks in some applications, however. For example, a large amount of time is sometimes required for the analog level to stabilize after switching. There is also no allowance for system gain control, thus adding large measurement inaccuracies unless precise temperature control is maintained.